2013‎ > ‎01‎ > ‎

WeWW! The Beautiful Colours of Hydrangea!

posted Dec 15, 2016, 11:42 PM by Grace Ong   [ updated Dec 15, 2016, 11:43 PM ]
The following post was first posted on Blogger on Tuesday, 8 January 2013.

pink and violet hydrangeas

Blue, violet, pink... and any colour in between. Ever wondered why the hydrangea macrophylla (a.k.a. bigleaf hydrangea) exhibits so many different hues?

Gardening websites like this one will tell you that low (acidic) soil pH yields blue bigleaf hydrangea flowers while high (alkaline) soil pH yields pink flowers. At moderate soil pH, violet blooms are observed.

From a chemical perspective, the colour variation is dependent on the concentration of aluminium ions in the flowers rather than on pH: a higher concentration of aluminium ions results in the blue colour being more accentuated.

Bigleaf hydrangea flowers contains an anthocyanin pigment, specifically, the delphinidin-3-glucoside, which can form a blue metal complex with aluminium ions. A high pH with high hydroxide ion concentration reduces the availability of aluminium ions, so the delphinidin-3-glucoside exists in a red 'uncomplexed' cationic form:


as its red cationic form
red hydrangea

On the other hand, a low pH releases aluminium ions which then form a blue complex with the anionic form of delphinidin-3-glucoside:

delphinidin-3-glucoside complexed with aluminium ion
as its blue anionic form
complexed with Al3+
blue hydra

At moderate pH, a mixture of cationic and complexed anionic forms of delphinidin-3-glucoside results in an intermediate violet colour in these hydrangea flowers.

Here are some gardening tips on how to change the colour of bigleaf hydrangea flowers by manipulating both pH and aluminium content in soil.
[Note: These tips are probably of no use to gardeners in hot, sunny Singapore.]